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I really can't beleive how many people here(and on oficial forums, but at least those are trolls)demand classic servers or say that vanilla was better. I agree, some things were fun, such as the period without battlegrounds and the period without battlegroups. On the other side, I think many players didn't even play in vanilla, and they just say things that heard from others for sake of conversations.
I, for one, remember:
-Bugged instances, such as abnormal spawns in BWL(Razorgore anyone? ), evade bugs, abnormal ability usages,boss phases not being properly changed and other things that made you pull your hair out, especially when the boss was almost done.
-Several bugged abilities(and often chainbugging other abilities when you used them)
-No DR in PvP and the overall value of gear made rank 13-geared players useless versus T2.5/T3 players(and T3 had roughly the same value as 70 blues). Doubletrinket mages could oneshot everyone, ambush rogues as well. An average mage would beat an extremely skilled warrior no matter what, since the warrior had no tools to kite all that kiting(hell,you could kill warrs with R1 Frostbolt if you wanted to).
-PvP was just a big grind. If you didn't spend your entire day on grinding mindless battlegrounds, with barely any skill involved.
-Most bosses were not complex. They were basically tank and spank with the odd ability and an enrage timer tuned around gear from the previous tank and spank instance. Indeed, there were some extremely hard fights, such as Four Horsemen, but no, Molten Core wasn't epic at all.
In my opinion, WoW was the best during TBC. Blizzard managed to create a really balanced PvP system and there was plenty of world PvP on my server. Arena was a pretty fun addition to the game. PvE bosses were extremely challenging, such as pre-nerf M'uru or pre-nerf Kael'thas, where a single mistake meant a sure wipe.
Now, WOTLK did screw up class balance pretty bad(mainly, through burst). Hardmodes are a nice addition to the game(YG-0, Algalon 25, Freya25 3E, and Mirmiron25 harmode are quite challenging), since several people complained about BC bosses being undoable if you didn't spend too much time/night trying it. However, normal modes are way too easy and I don't agree with Blizzard on this one.